1. Field of the Invention
This invention relates to a thermal head comprising a protection layer having a printing surface which is brought into contact with a heat sensitive record medium, a heat generating section which includes heat generating resistors and electrodes connected to the heat generating resistors and generates a heat to be transferred to said heat sensitive record medium through said protection layer, and a driving circuit connected to said electrodes for supplying a heating electric power to the electrodes.
The present invention also relates to a method of manufacturing the thermal head of the kind mentioned above.
2. Related Art Statement
The thermal head of the kind described in the preamble has been used in simple and low cost printers using heat sensitive papers and heat transfer papers which do not require a supply of inks. In a printer using such a thermal head, a high image quality and high printing speed have been required. For instance, in a heat transfer type color printer or an index printer installed in an automatic mini-laboratory, a thermal head having a very high resolution such as 600 dpi to 1200 dpi has been required.
However, in such a thermal head, an excellent heating up and cooling down property is required in order to raise a temperature of the heat generating section within a very short time and to dissipate a generated heat at a high rate. Such a high speed heating up and cooling down property is particularly required for avoiding undesired blur in an printed image. In order to attain a prompt heating up, it is required that a generated heat does not escape from the heat generating section, and in order to effect the rapid heat dissipation, a generated heat has to be dissipated as soon as possible. For attaining a desired heating up and cooling down property, these two contradictory problems have to be solved simultaneously.
Various requirements generally required for the thermal head may be summarized as follows.
1) small size, light weight, simple structure
2) low price
3) large image size covering A3 size
4) low power consumption
5) high printing speed
6) high density and high resolution
7) uniform image quality without irregular color
8) maintenance free faculty
FIG. 1 is a cross sectional view showing a known typical thermal head. This known thermal head comprises an alumina substrate 1 serving as a heat sink, a heat storage layer 2 formed by depositing a glass having a high melting point of about 1000xc2x0 C. on the alumina substrate 1, a heat generating resistors 3 formed on the heat storage layer 2, a common electrode 4a connected commonly to one ends of the resistors 3, separate electrodes 4b connected to the other ends of respective resistors 3, and a protection layer 5 formed on the resistors 3 and electrodes 4a, 4b. The protection layer 5 operates as an abrasion resistance layer and includes a printing surface against which a heat sensitive record paper 7 is urged by a press roller 6.
Upon manufacturing the heat storage layer 2, a paste prepared by adding glass powders to a binder is applied on the surface of the substrate 1, and then an assembly is heated to sinter the glass. In the known thermal head, the glass having a high melting point is used for the heat storage layer 2 due to the following reasons. When the electrodes 3 is made of a metal having a low electric resistance such as aluminum and copper, the glass of the heat storage layer 2 has to be sintered at a high temperature in order to prevent the electrodes from being oxidized or altered by the heat storage layer. Furthermore, in order to improve a resistance/temperature coefficient (TCR) of the heat generating resistors, the resistors have to be subjected to a relatively high heating treatment, and thus the glass of the heat storage layer should have a melting point higher that a temperature of said heating treatment. The heat storage layer 2 has also a function to compensate a roughness of the surface of the substrate 1.
As shown in FIG. 1, on the substrate 1, there is further secured a driving IC 8 by means of an adhesive layer 9, and the separate electrodes 4b are connected to the IC 8 by means of connecting wires 10.
In the conventional thermal head illustrated in FIG. 1, a shallow groove 5a is formed in the printing surface of the protection layer 5, and therefore the heat sensitive record paper 7 could not be effectively brought into contact with the printing surface. Therefore, a heat generated by the heat generating resistors 3 could not be efficiently transferred to the heat sensitive record paper 7 through the printing surface.
In order to mitigate the above mentioned problem of the known thermal head shown in FIG. 1, in Japanese Patent Kokai Hei 5-64905, there is proposed another known thermal head as shown in FIG. 2. In this known thermal head, a printing surface is formed to be flat, and therefore a space is hardly formed between the printing surface and a heat sensitive record medium and a thermal efficiency is improved. As shown in FIG. 2, on a flat surface of a preliminary substrate 11, a pealing-off layer 12, a wear and abrasion resistance layer 13, a protection layer 14, a heat generating resistance layer 15, an electrode layer 16 and a heat storage layer 17 made of a synthetic resin are successively applied. After cementing the assembly to a substrate 18 by means of an adhesive layer 19, the preliminary substrate 11 is removed by means of the pealing-off layer 12. In this manner, a flat printing surface 13a can be obtained.
However, in the known thermal head illustrated in FIG. 2, in which the heat storage layer 17 is made of a synthetic resin and the printing surface 13a constituted by a portion of the wear and abrasion resistance layer 13 is formed to be flat, has the following problem.
Since the heat storage layer 17 is subjected to a high temperature process and is liable to be softened. Therefore, the heat storage layer 17 is preferably made of a heat resistant synthetic resin such as epoxy resin and polyimide resin. However, it has been experimentally confirmed that although the heat storage layer 17 is made of a heat resistant synthetic resin, it is softened during the operation. When the heat resistant layer 17 is softened, it might be deformed to a certain extent. For instance, if a hard particle is introduced between the printing surface of the thermal head and the press roller or heat sensitive paper, the heat storage layer 17 might be locally deformed. Moreover, when the heat storage layer 17 is softened, its mechanical strength is decreased, and thus the thermal head might be deformed and a quality of a printed image might be deteriorated.
In the known thermal head illustrated in FIG. 2, since the heat storage layer 17 is formed after the heat generating resistor layer 15 and electrode layer 16 have been formed, the heat storage layer 17 could never be made of a glass having a high melting point like as the thermal head shown in FIG. 1. Therefore, the above mentioned problem of softening could not be avoided.
Moreover, in the known thermal head shown in FIG. 2, the heat sensitive record paper is urged against the thermal head with a very strong force by means of a press roller 6, but since the printing surface is flat, the roller is brought into contact with the thermal head over a larger area and thus the pressing force per unit area is decreased. This results in that an influence of the roller deformation and abrasion might occur.
In order to mitigate the above problems, the inventors have proposed a thermal head, in which a printing surface is curved outwardly or is protruded from one surface of a protection layer and a driving IC is provided on the other surface of the protection layer. In this thermal head having the protruded printing surface, the heat sensitive record paper is urged against the printing surface with an extremely large force. However, this introduces the above mentioned problems of undesired deformation of the heat storage layer and undesired decrease in an image quality.
The present invention has for its object to provide a novel and useful thermal head having a large mechanical strength and a high image quality.
It is another object of the invention to provide a thermal head having a printing surface against which a heat sensitive record medium can be urged at a large force and a heat can be efficiently transferred to the record medium without causing undesired deformation of the thermal head.
According to a first aspect of the invention, a thermal head comprises:
a protection layer having mutually opposed first and second surfaces, said first surface including a smooth printing surface which is brought into contact with a heat sensitive record medium;
a heat generating section provided on said second surface of the protection layer at a position corresponding to said printing surface and including heat generating resistors and electrodes connected to the heat generating resistors for generating heat to be transferred to said heat sensitive record medium through said printing surface of the protection layer;
a driving circuit connected to said electrodes of the heat generating section for supplying a heating electric power to the electrodes; and
a heat storage layer made of a glass having a low melting point and provided on a side of said heat generating section remote from said protection layer.
According to the invention, it is preferable to make the heat storage layer of a glass having a melting point within a range of 300-450xc2x0 C., particularly 350-400xc2x0 C.
In a preferable embodiment of the thermal head according to the first aspect of the invention, a barrier layer is provided between said heat generating section and said heat storage layer, said barrier layer serving to prevent an undesired diffusion of substances contained in the low melting point glass of the heat storage layer into the heat generating section.
According to the invention, the protection layer includes the smooth printing surface which is free from a step corresponding to a thickness of the electrodes, and the smooth printing surface may be generally formed to be flat or protruded from the remaining surface of the protection layer.
According to a second aspect of the invention, a thermal head comprises:
a protection layer having mutually opposed first and second surfaces, said first surface including a printing surface which is brought into contact with a heat sensitive record medium and is protruded from the remaining first surface of the protection layer;
a heat generating section provided on said second surface of the protection layer at a position corresponding to said protruded printing surface and including heat generating resistors and electrodes connected to the heat generating resistors for generating heat to be transferred to said heat sensitive record medium through said protruded printing surface of the protection layer;
a driving circuit connected to said electrodes of the heat generating section for supplying a heating electric power to the electrodes;
a reinforcing member provided on a side of said heat generating section remote from said protection layer; and
a supporting member made of a synthetic resin, said reinforcing member being covered with said supporting member.
In the thermal head according to the second aspect of the present invention, said reinforcing member may be made of an aggregate of ceramic particles, a glass or a rod lake member. In case of forming the reinforcing member by a glass, a glass dispersion is applied on the rear surface of the heat generating section by means of screen printing or dispenser and is then solidified again. Furthermore, said supporting member may be made of a heat resistant synthetic resin such as epoxy resin and polyimide resin containing inorganic particles such as alumina, silica and glass particles.
Furthermore, according to the thermal head of the present invention, although the reinforcing member serves as a heat storage member, a separate heat storage member may be provided between the heat generating section and the reinforcing member.
According to a third aspect of the invention, a thermal head comprises:
a protection layer having mutually opposed first and second surfaces, said first surface including a printing surface which is brought into contact with a heat sensitive record medium and is protruded from the remaining first surface of the protection layer;
a heat generating section provided on said second surface of the protection layer at a position corresponding to said protruded printing surface and including heat generating resistors and electrodes connected to the heat generating resistors for generating heat to be transferred to said heat sensitive record medium through said protruded printing surface of the protection layer;
a driving circuit connected to said electrodes of the heat generating section for supplying a heating electric power to the electrodes; and
a heat control section for controlling a temperature of said heat generating section and being provided on a side of said heat generating section remote from said protection layer.
In a preferable embodiment of the thermal head according to the third aspect of the invention, said heat control section comprises a heat storage layer, which may be made of a glass having a low melting point or a heat resistant synthetic resin such as epoxy resin and polyimide resin. In case of using the heat resistant synthetic resin, ceramic fillers or powders such as alumina and silica and/or metal powders may be added to the synthetic resin for adjusting a mechanical property and a thermal property of the heat storage layer.
In a preferable embodiment of the thermal head according to the third aspect of the invention, said heat control section further comprises a heat conduction member for dissipating a heat stored in the heat storage layer. By suitably constructing said heat storage layer and heat conduction member, the heat control can be performed optimally. In this manner, a rapid heating-up and a prompt cooling-down can be attained. Moreover, said heat control member may be made of a glass rod which also serves as a reinforcing member. Therefore, the mechanical strength of the thermal head can be improved. Further, said heat conduction member may be made of an alumina based ceramic coating agent.
In the thermal head according to the third aspect of the invention, an assembly of the protection layer, heat generating section, heat control section and driving IC may be supported by a supporting member. This supporting member may be formed by a heat resistant synthetic resin or a metal plate. In case of using the heat resistant synthetic resin, the driving IC may be embedded in the supporting member, and in case of using a metal substrate plate, the driving IC may be provided in a recess formed in the metal substrate plate.
According to the invention, said protection layer is preferably made of a material selected from the group consisting of SiC compounds, SiB compounds, SiN compounds, AlN compounds, BN compounds, SiBP compounds, SiBN compounds, SiBC compounds, BPN compounds and BCN compounds.
The present invention also relates to a method of manufacturing the thermal head, and has its object to provide a novel and useful method, by means of which the thermal head can be manufactured precisely and efficiently at a low cost.
According to a fourth aspect of the invention, a method of manufacturing a thermal head comprises the steps of:
forming a protection layer on a flat surface of a preliminary substrate;
forming a heat generating section on said protection layer, said heat generating section including heat generating resistors and electrodes connected to the resistors;
forming a heat storage layer made of a low melting point glass on said heat generating section such that said heat generating resistors and a part of said electrodes are covered with the heat storage layer; and
removing said preliminary substrate.
In such a method according to the invention, after forming the protection layer and heat generating section on the flat surface of the preliminary substrate, the heat storage layer is formed on a side of the heat generating section remote from the protection layer, and therefore the printing surface of the thermal head can be flat.
In the method according to the fourth aspect of the invention, a barrier layer may be formed between said heat generating section and said heat storage layer. Furthermore, said heat storage layer may be secured to a supporting member.
According to a fifth aspect of the invention, a method of manufacturing a thermal head comprises the steps of:
forming a groove in a surface of a preliminary substrate;
forming a protection layer on an inner surface of said groove as well as on said surface of the preliminary substrate, a portion of said protection layer provided on the inner surface of the groove constituting a printing surface;
forming a heat generating section on said protection layer at least at said groove, said heat generating section including heat generating resistors and electrodes connected to the resistors;
forming a reinforcing member on said heat generating section such that said heat generating resistors and at least a part of the electrodes are covered with said reinforcing member; and
removing at least a part of said preliminary substrate such that at least a part of said protruded printing surface of the protection layer is exposed.
In a preferable embodiment of the method according to the fifth aspect of the invention, said preliminary substrate may be removed completely or may be removed partially. In the latter case, the preliminary substrate removing step may include a step of covering an assembly of the preliminary substrate, protection layer and heat generating section with an anti-etching layer, and a step of etching a part of the preliminary substrate. In this case, at first, said preliminary substrate is mechanically polished to such a level that said printing surface is still covered with a thin film of a material of said preliminary substrate, and then, the preliminary substrate is wet-etched or chemically-mechanically polished until said printing surface is exposed.
Further, a heat sink made of a metal such as aluminum and copper may be provided in the substrate in order to improve the heat dissipation property.
Furthermore, in order to reduce a size of the thermal head, it is preferable that an IC constituting said driving circuit is arranged on a second surface of the protection layer, said second surface being opposite to said first surface. In this case, the IC may be embedded in a supporting member made of a resin or may be provided in a recess formed in the second surface of the reinforcing layer
or may be provided in a recess formed in the substrate.
According to the invention, said protection layer is preferably made of a material selected from the group consisting of SiC compounds, SiB compounds, SiN compounds, AlN compounds, BN compounds, SiBP compounds, SiBN compounds, SiBC compounds, BPN compounds and BCN compounds, SiBP compounds, SiBN compounds, SiBC compounds, BPN compounds and BCN compounds. In this case, it is preferable that said reinforcing layer is made of a glass such as borosilicate glass.
According to a sixth aspect of the invention, a method of manufacturing a thermal head comprises the steps of:
forming a groove in a surface of a preliminary substrate;
forming a protection layer on an inner surface of said groove as well as on said surface of the preliminary substrate, a portion of said protection layer provided on the inner surface of the groove constituting a printing surface;
forming a heat generating section on said protection layer at least at said groove, said heat generating section including heat generating resistors and electrodes connected to the resistors;
forming a heat control section on said heat generating section such that said heat generating resistors and at least a part of the electrodes are covered with said heat control section; and
removing at least a part of said preliminary substrate such that at least a part of said protruded printing surface of the protection layer is exposed.
According to the present invention, it is preferable to conduct said step of removing the preliminary substrate by mechanically polishing said preliminary substrate and by wet-etching or chemical-mechanical-polishing the preliminary substrate.